A circuit board comprising printed transmission lines or microstrips with associated ground planes not only provides compact and lightweight circuits, but also provides broadband frequency capabilities. This type of circuit board can be carried, for example, in a housing and is widely used, for example, as a microwave interconnect device for telecommunications and satellites. With such a microwave device, there frequently is a need to transition a coaxial cable to the microstrip to thereby launch microwave signals onto the circuit board, for example. The coaxial cable typically comprises an inner conductor and a shield conductor surrounding the inner conductor, and the transition may involve electrically connecting the inner conductor to the microstrip and connecting the shield conductor to a ground plane corresponding to the microstrip.
The ground plane is typically buried just beneath a surface of the circuit board as reference to the microstrip. Thus, a conventional approach to making the coaxial cable-to-microstrip transition, as typified by the Tri-Band Block Converter made by the assignee of the present invention, is to expose the ground plane by forming an opening in the surface of the circuit board. With the ground plane thus exposed, the shield conductor can be soldered to the ground plane. This approach may pose disadvantages, however. For example, providing an opening in the top layer of the circuit board to expose an underlying ground plane usually requires that the circuit board be constructed in multiple and cumbersome steps, which adds to the cost of manufacturing such circuit boards.
Another particularly vexing problem relates to the difficulty associated with accurately aligning the coaxial cable relative to the microstrip so that they may be properly connected, typically by soldering an end of the inner conductor to a corresponding end of the microstrip. Even when the coaxial cable is properly aligned, it still may be quite difficult to maintain the coaxial cable in position during soldering.
Despite the difficulty, it is important that the alignment of the coaxial cable and microstrip be accurately made and maintained. Otherwise, there may be considerable difference between the electrical lengths of the signal and return paths that result from the transition. Even modest differences in the signal and return paths may degrade the voltage standing wave ratio (VSWR), which, in turn, can cause amplitude ripple due to mismatching.
In recent years, several types of perpendicular coaxial cable-to-microstrip transitions have been proposed. For example, U.S. Pat. No. 5,886,590 to Quan et al. discloses an orthogonal coaxial cable-to-microstrip transition. The transition uses a compressible connector for providing a solderless contact between the inner conductor of a coaxial cable and a microstrip. U.S. Pat. No. 6,236,287 to Quann et al. similarly discloses a compressible interconnect that provides a connection between the inner conductor of a coaxial cable and a microstrip, the inner conductor and microstrip being perpendicular to one another.
Perpendicular coaxial cable-to-microstrip transitions, however, can be relatively complex and, accordingly, may be difficult to properly manufacture, thus adding to the cost of a microwave or other device requiring a circuit board with such a transition. More generally, these and other conventional coaxial cable-to-microstrip transitions may rely on a structure and/or components that extend too far off of the circuit board to achieve the narrow profile that may be needed for many applications. For these and other reasons, circuit devices relying on conventional coaxial cable-to-microstrip transitions may not be suited for certain types of applications.